The invention relates to a method of reducing metal oxide in a blast furnace using an untreated top gas and an oxygen-enriched hydrogenaceous fuel. More particularly, the invention includes introducing an untreated heated top gas into the lower half of a blast furnace stack and introducing an oxygen-enriched hydrogenaceous fuel into a blast furnace bosh.
The growth of so-called "minimills" has been rapid in recent years. One reason for this growth is because minimills produce steel by remelting ferrous scrap in an electric furnace rather than using molten pig iron produced in a blast furnace. On the other hand, most fully integrated large steel manufacturers require molten pig iron as a charge material for a steel converter such as a basic oxygen furnace. One of the primary reasons why blast furnaces no longer are being constructed in North America is because of the huge capital investment required. This not only includes capital for the furnace itself but also related equipment such as stoves for heating blast air and a coke producing oven. Another disadvantage is the maintenance expense associated with the stoves required to heat the blast air.
British patent 1,216,779 discloses a method of operating a blast furnace at a pressure of at least 75 psi to produce molten pig iron. Oxygen, steam and optional fuel oil are blown through blast furnace tuyeres. Carbon dioxide allegedly is removed from the top gas using a scrubber and the remaining gas is recycled to the base of the furnace at a pressure exceeding that within the furnace. The patent discloses the need for blast furnace stoves may be reduced or eliminated.
A major disadvantage associated with the operation of a blast furnace is the high energy cost associated with the use of coke in the furnace burden. The environmental problems associated with coke ovens for producing coke are well known. There has been much effort in recent years to reduce the consumption of coke and to increase the output of blast furnaces. Coke consumption in the furnace can be reduced by recycling the furnace generated top gas. Since top gas contains large amounts of carbon dioxide and water, this gas first must be regenerated with a carbon-containing material such as methane to make it more reducing before it can be injected into the furnace bosh. However, this requires the added capital expense of either a reformer or a gas separation unit. Accordingly, top gas generally is used for firing the stoves but not for reducing metal oxide in the furnace. Coke consumption also can be reduced by blowing carbonaceous and hydrogenaceous materials along with hot blast air into the furnace. A disadvantage of this process can be the large amount of heat required to combust the fuel to produce a reducing gas. The more endothermic the injected fuel, the more oxygen enrichment of the blast is needed to sustain the flame temperature at the tuyeres.
U.S. Pat. No. 4,884,677 discloses a method of operating a blast furnace to produce molten pig iron using an oxygen-enriched fuel and recycling regenerated top gas to the furnace. Top gas is reformed to carbon monoxide and hydrogen using a fossil fuel. The regenerated top gas, oxygen and fuel are blown through the bosh tuyeres. A portion of the regenerated top gas may be blown through the furnace stack. The temperature of the regenerated top gas may be raised to 1500.degree. C. and the temperature of the oxygen and fuel raised to 500.degree. C.
U.S. Pat. No. 4,844,737 discloses operating a blast furnace to produce molten pig iron using an oxygen-enriched fuel and recycling heated top gas to the furnace. A reducing gas containing at least 40% by vol. oxygen and pulverized coal having predetermined fuel and coal ratios are blown through the bosh tuyeres. A top gas having a temperature of 1000.degree. C. to heat the furnace burden is blown through inlets set in the furnace shaft portion.
U.S. Pat. No. 4,917,727 discloses a method of operating a blast furnace to produce molten pig iron using an oxygen-enriched fuel and recycling heated top gas to the furnace. Pure oxygen, pulverized coal and a temperature control gas are blown through the bosh tuyeres. A preheating gas is blown through an intermediate portion of the furnace stack to increase the gas flow within the furnace and to preheat the furnace burden. The temperature control gas and the preheating gas are top gas with the latter being combusted.
Nevertheless, there remains a long felt need to reduce the capital cost for the equipment associated with an efficient blast furnace operation. There further remains a need to reduce operating costs by reducing the amount of coke required in the operation of a blast furnace.